Produce targeted diagnostic when using doc comments on fn args

Before parsing argument names and types, try to consume an incorrectly
included doc comment or attribute in order to recover and continue
parsing the rest of the fn definition.

src/libsyntax/parse/parser.rs

@@ -1789,6 +1789,35 @@ impl<'a> Parser<'a> {
         self.look_ahead(offset + 1, |t| t == &token::Colon)
     }
 
+    /// Skip unexpected attributes and doc comments in this position and emit an appropriate error.
+    fn eat_incorrect_doc_comment(&mut self, applied_to: &str) {
+        if let token::DocComment(_) = self.token {
+            let mut err = self.diagnostic().struct_span_err(
+                self.span,
+                &format!("documentation comments cannot be applied to {}", applied_to),
+            );
+            err.span_label(self.span, "doc comments are not allowed here");
+            err.emit();
+            self.bump();
+        } else if self.token == token::Pound && self.look_ahead(1, |t| {
+            *t == token::OpenDelim(token::Bracket)
+        }) {
+            let lo = self.span;
+            // Skip every token until next possible arg.
+            while self.token != token::CloseDelim(token::Bracket) {
+                self.bump();
+            }
+            let sp = lo.to(self.span);
+            self.bump();
+            let mut err = self.diagnostic().struct_span_err(
+                sp,
+                &format!("attributes cannot be applied to {}", applied_to),
+            );
+            err.span_label(sp, "attributes are not allowed here");
+            err.emit();
+        }
+    }
+
     /// This version of parse arg doesn't necessarily require
     /// identifier names.
     fn parse_arg_general(&mut self, require_name: bool) -> PResult<'a, Arg> {
@@ -1797,7 +1826,11 @@ impl<'a> Parser<'a> {
         let (pat, ty) = if require_name || self.is_named_argument() {
             debug!("parse_arg_general parse_pat (require_name:{})",
                    require_name);
-            let pat = self.parse_pat()?;
+            self.eat_incorrect_doc_comment("method arguments");
+            let pat = self.parse_pat().map_err(|mut err| {
+                err.span_label(self.span, "expected argument name");
+                err
+            })?;
 
             if let Err(mut err) = self.expect(&token::Colon) {
                 // If we find a pattern followed by an identifier, it could be an (incorrect)
@@ -1819,10 +1852,12 @@ impl<'a> Parser<'a> {
                 return Err(err);
             }
 
+            self.eat_incorrect_doc_comment("a method argument's type");
             (pat, self.parse_ty()?)
         } else {
             debug!("parse_arg_general ident_to_pat");
             let parser_snapshot_before_ty = self.clone();
+            self.eat_incorrect_doc_comment("a method argument's type");
             let mut ty = self.parse_ty();
             if ty.is_ok() && self.token == token::Colon {
                 // This wasn't actually a type, but a pattern looking like a type,